1. Field of the Invention
This document relates to an organic light emitting diode display, and more particularly, to an organic light emitting diode display that adjusts the luminance of an output image depending on brightness of an input image, and a driving method thereof.
2. Discussion of the Related Art
Recently, various flat panel displays have been developed with lower weight compared with smaller than cathode ray tubes (CRTs). Flat panel displays include, for example, liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and electroluminescence devices.
Because the structure and manufacturing process of the PDP are simple, the PDP is spotlighted as a lightweight, thin, and small display and that is advantageous for use in large screen display applications. However, the PDP has low light emitting efficiency, low luminance and large power consumption. A thin film transistor LCD, in which a thin film transistor (hereinafter, “TFT”) is used as a switching device, is one of the most widely used flat panel displays. However, because the TFT LCD is a non-emitting device, the TFT LCD has a narrow viewing angle and low response speed. By contrast, electroluminescence devices are classified into inorganic light emitting diode displays and organic light emitting diode displays in accordance with material of an emission layer. In particular, the organic light emitting diode display has high speed, high light emitting efficiency, high brightness, and wide viewing angle by using a self-emitting device.
The organic light emitting diode display has an organic light emitting diode OLED as shown in FIG. 1. The organic light emitting diode includes an anode electrode, a cathode electrode, and organic compound layers that include the hole injection layer HIL, hole transport layer HTL, emission layer EML, electron transport layer ETL, electron injection layer EIL formed between the anode electrode and the cathode electrode.
When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the emission layer EML to form excitons. As a result, the emission layer EML generates visible light.
The organic light emitting diode display includes a plurality of subpixels arranged in a matrix, each subpixel including the organic light emitting diode. The organic light emitting diode display selects the subpixels by selectively turning on the TFTs, which are active elements, by a scan pulse, and controls the brightness of the selected subpixels in accordance with the gray scale of digital video data.
Such an organic light emitting diode display is susceptible to temperature. A larger display load results in a higher temperature, which affects the driving of the organic light emitting diode display. Temperature is an important factor in determining the life span and display quality of the organic light emitting diode OLED. Generally, the display load becomes much larger when displaying a bright image, rather than when displaying a dark image.
In the related art, there has recently been proposed a method in which the brightness of an input image is analyzed to produce peak luminance in the presence of an only partly bright image and to reduce luminance in the presence of an entirely bright image, thereby minimizing the load applied onto the organic light emitting diode OLED. The peak luminance makes white on a dark screen more distinct, and further improves picture quality. However, the proposed method has at least the following problems.
Firstly, in a related art, to determine the brightness of an input image, input digital video data is analyzed to extract maximum gray level value for each pixel, and then the extracted maximum gray level values are divided by a resolution to calculate an average gray level value in a corresponding frame. As a result, there is a limitation in reducing the size of the circuit logic because a division operation for dividing the maximum gray level values by a resolution is necessarily accompanied in the related art to calculate the average gray level value.
Secondly, in the related art, it is difficult to accurately reflect a situation of an image in luminance adjustment because the brightness of an input image is determined by using an average gray level value. For example, if the average gray level value is ‘127’, the gray level values of all pixels may be ‘127’, or otherwise half of them may be white gray levels and the other half may be black gray levels like a chess pattern. With the average gray level value taken as a reference, both of the two patterns go through the same processing, so there is a limitation in improving the picture quality of, especially, complex images.